Remove -no-cpp-precomp flag from Darwin builds

Remove -no-cpp-precomp flag from Darwin builds

GCC 9 no longer accepts this flag and trying to find out what it does do
leads to an article

"-no-cpp-precomp: the compiler flag that time forgot"

that also no longer on the Internet. And most other things are
PRs/commits from over ten years ago that remove the flag since it
is no longer needed.
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20190705114243.9481-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18650.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 0740e079a1c094627a69b8b5959c6be56e15d9f6)